Process Scheduling 30 Process Scheduling The Operating System Kernel Basic set of primitive operations and processes • { Primitive: Like a subroutine call or macro expansion ∗ Part of the calling process ∗ Critical section for the process ∗ { Process Synchronous execution with respect to the calling process ∗ Can block itself or continuously poll for work ∗ More complicated than primitives and more time and space ∗ Provides a way to provide protected system services, like supervisor call instruction • { Protects the OS and key OS data structures (like process control blocks) from interference by user programs { The fact that a process is executing in kernel mode is indicated by a bit in program status word (PSW) Set of kernel operations • { Process Management: Process creation, destruction, and interprocess communication; schedul- ing and dispatching; process switching; management of process control blocks { Resource Management: Memory (allocation of address space; swapping; page and segment management), secondary storage, I/O devices, and files { Input/Output: Transfer of data between memory and I/O devices; buffer management; allo- cation of I/O channels and devices to processes { Interrupt handling: Process termination, I/O completion, service requests, software errors, hardware malfunction Kernel in Unix • { Controls the execution of processes by allowing their creation, termination, suspension, and communication { Schedules processes fairly for execution on cpu cpu executes a process ∗ Kernel suspends process when its time quantum elapses ∗ Kernel schedules another process to execute ∗ Kernel later reschedules the suspended process ∗ { Allocates main memory for an executing process Process Scheduling 31 Swapping system: Writes entire process to the swap device ∗ Paging system: Writes pages of memory to the swap device ∗ { Allocates secondary memory for efficient storage and retrieval of user data { Allows controlled peripheral device access to processes Highest Level of User Processes: The shell in Unix • { Created for each user (login request) { Initiates, monitors, and controls the progress for user { Maintains global accounting and resource data structures for the user { Keeps static information about user, like identification, time requirements, I/O requirements, priority, type of processes, resource needs { May create child processes (progenies) Process image • { Collection of programs, data, stack, and attributes that form the process { User data Modifiable part of the user space ∗ Program data, user stack area, and modifiable code ∗ { User program Executable code ∗ { System stack Used to store parameters and calling addresses for procedure and system calls ∗ { Process control block Data needed by the OS to control the process ∗ Data Structures for Processes and Resources Process control block • { Most important data structure in an OS { Read and modified by almost every subsystem in the OS, including scheduler, resource allo- cator, and performance monitor { Constructed at process creation time Physical manifestation of the process ∗ Set of data locations for local and global variables and any defined constants ∗ { Contains specific information associated with a specific process The information can be broadly classified as process identification, processor state infor- ∗ mation, and process control information Process Scheduling 32 Process Id Identification Parent PID ! User Id User registers Program counter CPU state Condition codes ! Status info (interrupt enable flags) Stack pointer Process state Priority Process scheduling Wait-time ! Event being waited on Privileges Flags IPC Signals ! Messages Unit Owned/ Memory ! Description Shared !··· Resource Unit Owned/ Resources ! Class Description Shared !··· Created resources ! to# resource !··· descriptor Children Figure 1: Process Control Block Can be described by Figure 1 ∗ Identification. Provided by a pointer p^ to the PCB ∗ Always a unique integer in Unix, providing an index into the primary process table · Used by all subsystems in the OS to determine information about a process · Parent of the process · CPU state. ∗ Provides snapshot of the process · While the process is running, the user registers contain a certain value that is to be · saved if the process is interrupted Typically described by the registers that form the processor status word (PSW) or · state vector Exemplified by EFLAGS register on Pentium that is used by any OS running on · Pentium, including Unix and Windows NT Process State. Current activity of the process ∗ Running. Executing instructions. · Ready. Waiting to be assigned to a processor. · Process Scheduling 33 Blocked. Waiting for some event to occur. · Allocated address space (Memory map) ∗ Resources associated with the process (open files, I/O devices) ∗ Other Information. ∗ Progenies/offsprings of the process · Priority of the process · Accounting information. Amount of CPU and real time used, time limits, account · numbers, etc. I/O status information. Outstanding I/O requests, I/O devices allocated, list of open · files, etc. Resource Descriptors. • { Resource. Reusable, relatively stable, and often scarce commodity ∗ Successively requested, used, and released by processes ∗ Hardware (physical) or software (logical) components ∗ { Resource class. Inventory. Number and identification of available units ∗ Waiting list. Blocked processes with unsatisfied requests ∗ Allocator. Selection criterion for honoring the requests ∗ { Contains specific information associated with a certain resource p^.status_data points to the waiting list associated with the resource. ∗ Dynamic and static resource descriptors, with static descriptors being more common ∗ Identification, type, and origin ∗ Resource class identified by a pointer to its descriptor · Descriptor pointer maintained by the creator in the process control block · External resource name unique resource id · ) Serially reusable or consumable? · Inventory List ∗ Associated with each resource class · Shows the availability of resources of that class · Description of the units in the class · Waiting Process List ∗ List of processes blocked on the resource class · Details and size of the resource request · Details of resources allocated to the process · Allocator ∗ Matches available resources to the requests from blocked processes · Process Scheduling 34 Uses the inventory list and the waiting process list · Additional Information ∗ Measurement of resource demand and allocation · Total current allocation and availability of elements of the resource class · Context of a process • { Information to be saved that may be altered when an interrupt is serviced by the interrupt handler { Includes items such as program counter, registers, and stack pointer Basic Operations on Processes and Resources Implemented by kernel primitives • Maintain the state of the operating system • Indivisible primitives protected by \busy-wait" type of locks • Process Control Primitives • { create. Establish a new process Assign a new unique process identifier (PID) to the new process ∗ Allocate memory to the process for all elements of process image, including private user ∗ address space and stack; the values can possibly come from the parent process; set up any linkages, and then, allocate space for process control block Create a new process control block corresponding to the above PID and add it to the ∗ process table; initialize different values in there such as parent PID, list of children (initialized to null), program counter (set to program entry point), system stack pointer (set to define the process stack boundaries) Initial CPU state, typically initialized to Ready or Ready, suspend ∗ Add the process id of new process to the list of children of the creating (parent) process ∗ r0. Initial allocation of resources ∗ k0. Initial priority of the process ∗ Accounting information and limits ∗ Add the process to the ready list ∗ Initial allocation of memory and resources must be a subset of parent's and be assigned ∗ as shared Initial priority of the process can be greater than the parent's ∗ { suspend. Change process state to suspended A process may suspend only its descendants ∗ May include cascaded suspension ∗ Process Scheduling 35 Stop the process if the process is in running state and save the state of the processor ∗ in the process control block If process is already in blocked state, then leave it blocked, else change its state to ready ∗ state If need be, call the scheduler to schedule the processor to some other process ∗ { activate. Change process state to active Change one of the descendant processes to ready state ∗ Add the process to the ready list ∗ { destroy. Remove one or more processes Cascaded destruction ∗ Only descendant processes may be destroyed ∗ If the process to be \killed" is running, stop its execution ∗ Free all the resources currently allocated to the process ∗ Remove the process control block associated with the killed process ∗ { change priority. Set a new priority for the process Change the priority in the process control block ∗ Move the process to a different queue to reflect the new priority ∗ Resource Primitives • { create resource class. Create the descriptor for a new resource class Dynamically establish the descriptor for a new resource class ∗ Initialize and define inventory and waiting lists ∗ Criterion for allocation of the resources ∗ Specification for insertion and removal of resources ∗ { destroy resource class. Destroy the descriptor for a resource class Dynamically remove the descriptor for